1. Field of the Invention
The present invention relates, in general, to electro-magnetic acoustic transducers and, in particular, to a new and useful preamplifier and impedance matching circuit for electromagnetic acoustic transducers.
2. Description of the Related Art
Electromagnetic acoustic transducer (EMAT) systems have been developed and are used in an increasing number of applications, such as nondestructive examinations through many industries. EMATs perform ultrasonic inspections without the use of a fluid couplant which is required with conventional piezoelectric ultrasonic transducers. Inspection with EMATs is faster and more reliable than conventional transducers; and EMAT inspection is easy to automate.
A majority of EMAT applications use meanderline coils to generate acoustic waves directly in the metal components being inspected. Optimum response from EMATs which use these coils requires the generation of a burst of radio frequency (RF) alternating current to induce the ultrasonic waves at the coil. Peak currents of 100 to 200 amperes are required for the most effective use of the meanderline EMAT coils. The RF tone burst is presently provided by a commercially available RF power amplifier. Since these amplifiers have an output impedance of 50 ohms, an impedance matching circuit is required to generate the highest levels of excitation current. The EMAT instrumentation also performs the function of amplifying, filtering and detecting the response signals which occur when ultrasonic waves are reflected back to the EMAT coil due to defects.
If an application requires that the EMAT has to be used at distances of more than a few feet from the EMAT instrumentation, the sensitivity of the system can be decreased substantially. This decrease in sensitivity can be attributed to several factors including cable losses, less than optimum impedance matching and electromagnetic interference from external sources.
Presently, there is no known device which eliminates these above-listed deficiencies.